Abstract
The particle–gas interface in aerosol systems is of essential importance because it is here that many key atmospheric processes occur. In this study, we employ ambient pressure X-ray photoelectron spectroscopy (APXPS) to investigate the surface properties and processes of an atmospherically relevant carboxylic salt, sodium acetate, at subdeliquescence conditions. From the depth profiles of the elemental ratios of sodium, oxygen, and carbon, we find that after deliquescence–efflorescence cycles the salt surface is sodium-depleted. The mechanism of the observed depletion is proposed to be (i) the formation of neutral acetic acid in the solution due to the nature of the basic salt; (ii) the selective surface enhancement of neutral molecules under aqueous condition; and (iii) a hypothetical kinetic barrier to re-homogenization due to spatial separation and special local conditions on the surface, resulting in varied local surface composition. When the relative humidity gradually increases and approaches the d...
Highlights
Particle−gas interfaces are of essential importance in aerosol systems, and they directly determine many atmospheric processes,[1,2] such as surface-catalyzed chemistry,[3,4] halogen chemistry,[5] trace gas uptake,[6] new particle formation,[7] ozone depletion,[8] and heterogeneous ice nucleation.[9]
Once mounted in the experimental chamber, a complete deliquescence−efflorescence cycle was performed by increasing the environmental cell relative humidity (RH) to 46% (>deliquescence relative humidity (DRH)), followed by decreasing the RH to 3% to remove the majority of the adsorbed water
Note that even at RH = 3% there is a small component at 533.5 eV, which is either the H2O of the hydrate salt or the adsorbed water remaining after dehydration
Summary
Particle−gas interfaces are of essential importance in aerosol systems, and they directly determine many atmospheric processes,[1,2] such as surface-catalyzed chemistry,[3,4] halogen chemistry,[5] trace gas uptake,[6] new particle formation,[7] ozone depletion,[8] and heterogeneous ice nucleation.[9]. Particle−gas equilibria are well defined by classical thermodynamic theories like the Köhler theory, which describes the equilibrium water vapor pressure over soluble particles as a function of water activity and particle size. These simplified concepts become uncertain when considering microscopic systems like molecular adsorption on the outermost layer of particles. Surfacesensitive studies reveal that salt surfaces begin to adsorb water at RH well below the bulk phase transition,[16−22] leading to a predeliquescence solvation of the surface.[23−25] these initial surface changes can be critical for atmospheric processes due to their direct relevance for heterogeneous atmospheric chemistry.[14]
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